1. Field of the Invention
The present invention relates to a position detector for detecting a designated position on a tablet in which a plurality of antennas are vertically and horizontally arranged, as a matrix.
2. Related Arts
A position detector is known which includes a plane member, called a tablet, in which a plurality of antennas are arranged as a matrix and a touch pen used for selecting a position on the tablet, and which can detect the position on the tablet which is selected by using the touch pen.
FIGS. 6A and 6B are diagrams showing an example where the position detector is applied for an electronic apparatus used as an educational toy for children. In FIG. 6A, a toy 1 includes a book tablet 2a and a front tablet 2b,and a specific picture book 4, which is placed on the book tablet 2a. When operating the toy, a player uses the touch pen 5 to select a FIGURE, character or symbol in the picture book 4, pressing the touch pen 5 against or bringing it near an illustration. Then, as is shown in FIG. 6B, the selected figure, etc., is displayed on a monitor 6.
Further, when the player uses the touch pen 5 to draw a desired FIGURE or character on the front tablet 2b, the locus of the touch pen 5 is displayed on the monitor 6.
FIG. 7 is a diagram for explaining the principle employed for the detection of a position on the tablet 2 selected by using the touch pen 5. As is shown in FIG. 7, the tablet 2 includes a plurality of horizontally arranged antennas 20X and a plurality of vertically arranged antennas 20Y. Under the control of a detection controller 9, an antenna driver 8 drives the horizontal antennas 20X in the order from right to left, and the vertical antennas 20Y in the order from top to bottom, which sequentially radiate radio waves.
The radio waves radiated by the antennas are received at a reception antenna (not shown) incorporated in the touch pen 5. Of the level of the received radio waves, the strongest level is that radiated by the antennas which is nearest to the position selected using the touch pen 5. By comparing the levels of received waves and identifying the strongest level in the vertical and the horizontal directions, the detection controller 9 can detect a selected position.
FIG. 8 is a block diagram illustrating the touch pen 5. In FIG. 8, radio waves radiated by the antennas on the tablet 2 are received by a reception antenna 51 incorporated in the tip of the touch pen 5. A reception signal is amplified by a buffer amplifier 52, and the resultant signal is passed through a ceramic filter 53. The ceramic filter 53, which permits only a single frequency to pass and blocks all others, performs a noise removal function.
The reception signal is then converted by a comparator included in an analog circuit 54 into a level signal which is compared with a predetermined threshold level. The level signal is passed through a delay time adjustment circuit 55 and a connector to a calculation circuit (not shown) for detecting a position selected with the touch pen 5.
The delay time adjustment circuit 55 is provided to perform following functions. A variable delay time is provided for the ceramic filter 53 incorporated in the touch pen 5, and therefore, if a plurality of touch pens 5 are used to select the same position on the tablet 2, the delay times for the respective incorporated ceramic filters 53 will vary, and different positions will be detected. For example, even when the center of the tablet 2 is selected by using the touch pens 5, the detected position will be shifted in the vertical and horizontal directions in which the antennas are driven. Thus, since the delay times for the ceramic filters 53 vary, the distance shifted will differ for the individual touch pens 5. In order to prevent such an inconvenience, the delay time adjustment circuits 55 make an adjustment for the a variance in the delay times for the ceramic filters 53, so that a constant delay time is provided for all the touch pens 5.
However, the provision of the additional delay time adjustment circuit 55 increases the cost of a touch pen 5, and accordingly, the overall cost of a position detector is increased.
It is, therefore, one objective of the present invention to provide an inexpensive position detector for which the delay time for a ceramic filter need not be adjusted, i.e., an apparatus for which a delay time adjustment circuit is not required.
It is another objective of the present invention to provide a position detector which, without delay time adjustment circuits being required, can accurately detect a position when that position is simultaneously selected with a plurality of touch pens.
To achieve the above objectives of the present invention, there is provided a position detector comprising:
a member having a surface and a plurality of vertically and horizontally arranged antennas;
a controller for sequentially driving the vertically arranged antennas in vertical positive and vertical negative directions, and for sequentially driving the horizontally arranged antennas in horizontal positive and horizontal negative directions so that the antennas sequentially radiate radio waves;
a receiver for receiving the sequentially radiated radio waves on the surface of the member; and
a detector for detecting the position of the receiver on the surface of the member based on the levels of the received radio waves.
Preferably, the detector detects the position of the receiver as an average position for a first position detected based on the levels of the radio waves which are sequentially radiated in the vertical positive and horizontal positive directions, and a second position detected based on the levels of the radio waves which are sequentially radiated in the vertical negative and horizontal negative directions.
With this arrangement, variances relative to the position of a receiver which could be picked up by the detector are eliminated, without an adjustment by a delay time adjustment circuit being required, and the position of the receiver can be accurately detected.